Assessment of GRADE
We examined the certainty (quality) of evidence for outcomes using GRADE approach.15 The certainty of evidence was classified as four levels: high, moderate, low, and very low. Two reviewers considered comprehensively five factors (risk of bias, inconsistency, indirectness, imprecision, and publication bias) to assess the certainty of evidence for the following outcomes: perineal trauma, episiotomy, first- and second-degree perineal laceration, severe perineal laceration, duration of second-stage labor. The evidence could be downgraded from ’high’ by one level for serious (or by two levels for very serious) limitations.